The present invention, which is a "subject invention" under INTELSAT Contract No. Intel-683 (INTERSAT VII-A), relates generally to the field of energy cells such as batteries and fuel cells, and more particularly to such energy cells which employ pressure vessel type cells such as those currently used in earth orbiting satellites, and in other applications which employ pressure-vessel type cells.
It is well known that a plurality of electrical cells can be connected in series to obtain an electrical output from a battery consisting of the cells which will have a voltage equal to the sum of the voltages of the individual cells. However, the problem exists in such an arrangement that if one cell in the series is defective or is damaged during use, which results in the cell failing in an open circuit condition, the entire battery is rendered useless because it is impossible to pass an electric current through the battery for either charging or discharging purposes.
It is also well known that communications satellites utilize batteries to provide electric power for a variety of purposes, principally in connection with the operation of electronic communications and instrumentation equipment. Such satellites are typically put into a geo-synchronous orbit in which the satellite maintains a fixed location in space relative to a given position on the earth. But regardless of the nature of the orbit, a satellite is virtually isolated from the earth and from contact with man, since it is such an extremely complicated and expensive undertaking to put men and equipment into space to repair a satellite that it is virtually never done. Under these conditions, it is apparent that a battery failure can render the entire satellite useless, which can result in tremendous loss in terms of the cost of the satellite, the expense involved in the launching and the lost communications capability. It would not be unreasonable to assume that a battery failure in a communications satellite could result in a multi-billion dollar loss.
Other applications, such as electric automobiles and generators, and other applications where battery and/or fuel cell failure causes great harm or inconvenience, are candidates for the invention whenever the batteries or fuel cells employ high pressure cells such as nickel-hydrogen, silver-hydrogen, hydrogen-zinc, chloride-bromide, and sealed hydrogen-oxygen.
Thus, from the inception of communications satellites, there has been a continuing effort to improve the reliability of satellite batteries. This effort has met with considerable success, particularly with the development of the electrochemical cell which can operate over a wide range of ambient temperatures, has a relatively high energy density and can be constructed in a variety of configurations, all of which characteristics contribute to rendering this cell ideal for use in outer space. However, some electrochemical cells such as nickel hydrogen cells, by their nature, will only operate in a high pressure environment, typically in the range of 700 to 900 PSI when fully charged, and there must be confined in a pressure vessel which can withstand such internal pressures.
Unfortunately, it is this requirement of such electrochemical cells that renders them vulnerable to a hostile condition of outer space over which man's technological ability has little or no effect, and that is bombardment by meteorites which damage the pressure vessel, causing holes or cracks so that it leaks or completely ruptures. The loss of pressure within the vessel is the only known failure mechanism which results in an open circuit cell and therefore renders it useless since it will not pass electric current therethrough.
Open circuit protection devices currently available consist of one large diode in the "forward" direction to pass the load current, and several diodes in series in the "reverse" direction to allow recharging of the remaining cells to occur. For high current batteries, in the range of 50 to 100 amperes, such as are becoming common for synchronous orbit communications satellites, the "forward" diode dissipates a large amount of power under discharge conditions because of its in inherent 0.6 to 0.7 volt drop. At 100 amperes, the power dissipation is 60 to 70 watts. The thermal design, and accompanying mass, of providing such a circuit for every cell in a battery are very undesirable.